Which method is not effective in distinguishing between fat mass and fat-free mass?

Body mass index (BMI) is a method that simply takes an individual's weight and divides it by their height squared to provide a numerical value that categorizes them as underweight, normal weight, overweight, or obese. However, BMI does not differentiate between fat mass and fat-free mass, such as muscle, bone, or water. Given that it does not provide information about body composition and cannot identify how much of a person's weight is attributed to fat versus lean mass, it is ineffective for this specific purpose.

In contrast, methods like skinfold measurements, bioelectrical impedance analysis, and hydrostatic weighing are designed to provide more detailed insights into body composition. Skinfold measurements use calipers to assess the thickness of skinfolds at various sites on the body, which helps estimate body fat percentage. Bioelectrical impedance analysis sends a low-level electrical current through the body to estimate body composition based on resistance, differentiating between fat and lean mass. Hydrostatic weighing involves measuring body density and can accurately distinguish between fat mass and fat-free mass.

Thus, among the choices provided, BMI is not an effective method for distinguishing between fat mass and fat-free mass due to its reliance solely on weight and height, without accounting for the composition of that weight.